The Complex Interplay of Duration of Residence and Region of Birth on Perinatal Outcomes Among Migrant Women in Australia: A Cohort Study.

BACKGROUND: Although migrants are recognised as heterogeneous, research often emphasises origin over receiving-context experiences. OBJECTIVES: We investigated maternal risk factors and perinatal outcomes by duration of residence in Australia, stratified by region of birth. METHODS: This population-based cohort study using Victorian Perinatal Data Collection data included singleton births between 2017-2019 (N = 199,935). Exposures were maternal region of birth (United Nations-grouped countries) and duration of residence (< 2, 2-5, 6-10, > 10 years, [~15% missing]). Outcomes were grouped into preconception health (smoking, body mass index (BMI), pre-existing conditions, social disadvantage), structural factors (antenatal care, early ultrasound) and pregnancy/perinatal outcomes (gestational diabetes, hypertensive disorders, gestation at birth, stillbirth). Relative risks were estimated using robust Poisson regression, adjusted for maternal age and birth year, stratified by region of birth. RESULTS: Among the 36.7% of women born outside of Australia, most were from South Asia or South-East/East Asia, with a mean residence of 10 years. Compared with Australia-born women, migrant women generally had a lower risk of smoking, pre-existing hypertension, BMI ≥ 25, preeclampsia and preterm birth-although advantages were less pronounced in women residing longer and varied by region. Some groups had a higher risk of gestational diabetes (Oceania, South Asia, South-East/East Asia, West Asia) and stillbirth (Oceania, South Asia, Africa), with risks persisting regardless of residence duration. Late initiation of antenatal care was consistently observed among women born in New Zealand, Oceania, Africa, West Asia and Other regions. Sensitivity analyses (i) treating missing duration as a category; (ii) restricting to women migrating ≥ 18 years; and (iii) imputing missing data for duration, BMI and smoking, yielded similar results. CONCLUSIONS: Perinatal outcomes varied by region of birth and duration of residence, indicating the importance of both origin and receiving-context factors. Improving outcomes requires addressing structural barriers and upstream determinants disproportionately affecting specific migrant groups.